Continuing advances in information handling technology have led to the requirement for data storage and retrieval systems capable of handling extremely large volumes of information. Optical recording, in particular optical disc recording, allows recording and accessing of information at very high data rates with a much greater recording density and archivability than is possible with magnetic recording. A highly focussed laser beam is utilized to record and recover information on the optical recording media. The selection and alignment of diode lasers into an optical recording system is discussed by Bartolini et al. in I.E.E.E. Journal of Quantum Electronics, 1981, p. 69, and both read and write apparatus are disclosed in British Patent Application No. 2 016 747A.
Many types of recording media have been disclosed for laser writing and these can be divided into two basic sorts: those which require processing after writing, and those which can be read immediately after writing. It is the latter type, possessing "direct read after write" capability and commonly known as "DRAW" media, which are of particular interest.
In order to be useful as a light absorbing layer of the recording element, materials must be able to be applied to a substrate in the form of a thin, smooth layer of high optical quality and predetermined thickness. The materials must absorb at the frequency of the optical source. Various materials have been proposed for the recording media of DRAW systems, including, for example, thin metal films, metal-impregnated polymers and organic dyes. In these cases the laser beam provides a pulse of heat energy to the recording medium which causes a change in surface morphology; i.e., formation of a bump or crater, by ablation, vaporization or melting.
One type of DRAW media comprises thin metal films and, of these, tellurium containing mixtures as disclosed in Lou et al., J. Vac. Sci. Technol., 1981, 18, 78 have been widely used. However, the preparation of recording elements incorporating tellurium is by a relatively expensive vacuum sputtering technique in which the metal does not adhere well to the substrate. It also presents environmental complications because of the toxicity of the metal.
Examples of the use of metal-impregnated polymers in recording elements include the silver-impregnated gelatin systems disclosed in United States Patent No. 4 278 758. Greater sensitivity is claimed for these systems than for the tellurium films, but high concentrations of expensive silver are used in the recording medium.
An alternative type of DRAW media uses organic compounds in place of expensive metals. As well as providing advantages of cost, the thermal properties of organic compounds are generally superior since they possess low thermal conductivity and low melting/ decomposition temperatures. With the use of such systems it is important that the absorption of dye therein corresponds as closely as possible to the emission of the recording laser. Of the various lasers available, semiconductor laser diodes have the advantages, (over conventional gas lasers) of low cost, smaller size, and the possibility of easy signal modulation. The problem is, therefore, one of finding particular organic materials which have all the requisite physical properties and absorb strongly in the region compatible with laser diodes; i.e., the near infrared region of the spectrum, wavelengths between 700 and 1400 nm. Examples of dye-containing recording media for optical data storage are disclosed in Jipson and Jones, J. Vac. Sci. Technol., 1981, 18, 105; European Patent Application No. 79 200 789; Crowly et al., IBM Technical Disclosure Bull, 24, No. 11B, 1982; Law et al., Appl. Phys. Lett., 1981, 39, 718; U.S. Pat. Nos. 4 270 130, 4 364 986 and 4 446 223, PCT Patent Publication Nos. WO84/02794 and WO84/02795 and Japanese patent publication Nos. 57 203 237, 57 210 893, 57 210 894, 58 053 489, 58 056 894, 58 056 895, 58 077 043, 58 112 792, 58 219 090, 58 222 451, 58 224 447, 59 005 095, and 59 055 795.
The use of organic dyes in optical data storage systems has however, encountered some problems since the dyes tend to crystallize once applied to the substrate in thin layers, with an accompanying reduction in medium performance. This problem has been resolved, in the main part, by the coating of dye in a polymeric binder with a resultant elimination of significant detrimental crystallization.
The objective in fabricating a dye-polymer optical recording element is to provide a smooth, blemish-free coating with a high optical density at the output wavelength of the laser. Hence the binder should be able to accommodate a high loading of dye without allowing crystallization of the latter. Likewise, formation of the dye-polymer coating should not involve exposure to high temperatures or short-wavelength radiation, as these can cause dye bleaching, or distortion of the coating and/or substrate. The binder must also disintegrate to volatile fragments on absorption of the laser energy.
There is no specific teaching in the art as to the type of binder which is most suitable for use in dye-polymer optical recording media. In general, dye-polymer ablative systems are said to be superior to absorbing metal systems because of their low conductivity, low decomposition temperature and low melting point. Polymeric binders with low melting points are exemplified together with binders of low or oligomeric molecular weights, as imparting high or increased sensitivity to the media. However examples of the thermoplastics having higher glass transition temperatures, e.g. polyesters, polycarbonates, poly(N-vinyl carbazole), appear in general lists of possible binders. The most commonly-used binders are cellulose derivatives, especially nitrocellulose, thereby obeying the guidelines of low decomposition/melting point and lower binder molecular weight.
Thus much of the research into dye-in-binder systems has been concerned with the ablation properties of dye-polymer films. However, in practice, the major problem encountered in the use of dye-polymer media is not that ablation does not take place, but rather that on being repeatedly read, particularly when using a "read laser" of the same wavelength as the "write laser", the carrier to noise ratio is unacceptably degraded. To reduce the cost of the write/read device it is conventional that only a single laser is used, therefore, although the read laser power is only a fraction of the write power it is inevitable that absorption of the incident light during read will occur. This problem becomes greater as the optical density of the film is increased. Thus improvements in write-sensitivity obtained by increasing the optical density have always been accompanied by deterioration of the read-stability. The problem area in repeatedly-reading the media has been identified by mathematical modelling as the high temperatures involved in the process.
These calculations have revealed that the temperatures reached during ablation are as high as 2,000.degree. C. and during read they may reach 400.degree. C. (1 mW, 150 ns dwell time, 1 micron pit).
Our copending British Patent Application No. 8713563, filed June 10th 1987 discloses that dye-polymer optical recording media in which the binder is based on poly(acenaphthylene) exhibit improved repeated-reading properties compared to many known systems. We have now discovered that even greater repeated-read stability is imparted by cross-linked binders. Furthermore, this advantage is obtained with no impairment of the write-sensitivity.
Japanese Patent Publication Nos. 59 203 247, 59 201 244, 59 201 243, 59 201 241, 59 198 193 and 59 190 895 disclose optical data storage media in which the binder is formed by cross-linking a resin having a functional group in the presence of a metal-based cross-linking agent, generally with a dye having a functional group. The cross-linking agent is generally an alkoxide or chelate of a transition metal. The recording media are said to possess good shelf life and high temperature stability after recording, maintaining the initial signal/noise (S/N) ratio. These metal-cross-linked binders require high temperatures to effect the cure, risking degradation of the dye. Also the presence of heavy metal atoms is likely to inhibit the degradation of the recording medium to volatile fragments.
DE 3238285 describes photo-curable acrylic binders for optical recording media, but these require exposure to intense ultra-violet radiation, which is detrimental to many of the dyes suitable for dye-polymer recording media. Furthermore, as will be demonstrated later, the stability of dyes towards ambient light exposure can be markedly affected by the polymeric medium in which they are dispersed, and vinyl polymers have undesirable properties in this respect.
Japanese Patent Publication No. 61 184 535A discloses an optical recording medium comprising a molecule which is isomerised due to light and a highly cross-linked binder matrix e.g. thermosetting polyester, alkyd and melamine resins. The medium utilizes photochemical hole burning to obtain multiple recording.
It has now been found that binders which thermally cross-link may be used in optical recording systems comprising dye-polymer ablative recording media to provide excellent stability for repeated-read.